Field of the Invention
The present invention relates to a water heater, and particularly to a water heater of a latent heat recovery type capable of heating water by recovering latent heat of combustion gas.
Description of the Background Art
A water heater of a latent heat recovery type includes a secondary heat exchanger recovering mainly latent heat in addition to a primary heat exchanger recovering mainly sensible heat of combustion gas, and can obtain heat of condensation (latent heat) by condensing vapor contained in combustion gas. Thus, the water heater of the latent heat recovery type can achieve high heat exchange efficiency.
In such a water heater of the latent heat recovery type, drainage water (condensate) resulting from condensation of vapor in combustion gas within the secondary heat exchanger is generated. Here, combustion gas contains a nitrogen oxide generated as a result of reaction between nitrogen and oxygen in air caused by combustion or a sulfur oxide generated as a result of reaction between a sulfur component in fuel and oxygen caused by combustion. Thus, generated drainage water exhibits strong acidity owing to such a nitrogen oxide or sulfur oxide.
Drainage water generated in the secondary heat exchanger is normally discharged to the outside through a drainage water discharge path after it is neutralized by a neutralizer disposed in the drainage water discharge path. In the water heater including such a drainage water discharge path, a flow path for combustion gas and the drainage water discharge path communicate with each other.
Therefore, in a case of the water heater of the latent heat recovery type adapted to an exhaust suction system which suctions combustion gas and emits combustion gas to the outside, a fan is placed downstream of the secondary heat exchanger. Therefore, a pressure in the secondary heat exchanger becomes negative due to a fan suction pressure, outside air flows back into the secondary heat exchanger through the drainage water discharge path, and discharge of drainage water from the secondary heat exchanger may become difficult.
Then, in such a water heater, an approach for solving the problem above, for example, by providing a plurality of chambers in a drainage water tank provided in the drainage water discharge path and filling a hole allowing communication between the chambers with drainage water to thereby establish water seal has been adopted. A water-seal structure of a drainage water tank in such a water heater of a latent heat recovery type is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2009-172535 and Japanese Patent No. 5367603.
In a water heater of a latent heat recovery type including a drainage water tank having a water-seal mechanism as described above, measures have been taken against clogging of the drainage water discharge path. Specifically, measures such as a sensing member sensing rise of a fluid level in the drainage water tank is provided around a top portion of an internal space in the drainage water tank so as to sense rise of a fluid level in the drainage water tank due to clogging of the drainage water tank, and the water heater is stopped when it is the case. In a water heater of an exhaust suction type including a drainage water tank having a water-seal structure, however, in spite of absence of clogging of the drainage water tank or a drainage water discharge portion, a fluid level in the drainage water tank tends to rise due to a fan suction pressure, and such a defective condition as erroneous sensing by a sensing member and resultant stop of the water heater may be caused.